Sleep disturbance, which is often observed among patients with diabetes (1), is possibly caused by impaired glucose metabolism or physical and psychological discomfort due to the disorder. In addition,
a recent prospective study of women has indicated an interesting association between sleep patterns and later-onset type 2
diabetes, with a greater incidence among both short-term (<6 h) and long-term (>8 h) sleepers (2). Disturbance in sleep quality may also affect the later onset of overt diagnosis of type 2 diabetes. We investigated the
association between sleep disturbance and the subsequent onset of type 2 diabetes in a group of Japanese male employees.

We analyzed the database of an 8-year prospective study of male employees of an electrical company in Japan (3). We followed 2,649 male employees with no medical history of diabetes or other chronic illnesses at baseline for 8 years
from 1984 to 1992. Data from 2,265 (86%) male respondents, who were thoroughly followed, were analyzed. All subjects received
a medical checkup once a year during the follow-up to identify those with type 2 diabetes according to World Health Organization
criteria (4). A mailed questionnaire was used to assess sleep disturbance in the previous month at baseline. Two single-item questions
were asked concerning difficulty initiating sleep (“Did you have trouble falling asleep?”) and difficulty maintaining sleep
(“Did you often wake up in the middle of the night?”). The subjects were classified into one of two categories: low for those
who indicated “seldom” or “sometimes” and high for those who indicated “often” or “almost everyday” in response to the questions.

During the 18,006 person-year observation, 38 incidents of type 2 diabetes were identified (an incidence rate of 1.68 per
1,000 person-years). Those who experienced a high frequency of difficulty initiating sleep had a significantly higher age-adjusted
hazard ratio (2.98, 95% CI 1.36–6.53) for type 2 diabetes compared with those who experienced low-frequency difficulty initiating
sleep. A similar hazard ratio was observed for difficulty maintaining sleep (2.23, 1.08–4.61). These hazard ratios were almost
identical and were statistically significant after controlling for other factors relevant to type 2 diabetes (i.e., age, education,
occupation, shift work, BMI, leisure-time physical activity, smoking, alcohol consumption, and family history of diabetes).

Our analysis demonstrated that those who had sleep disturbances showed a two- to threefold higher risk of later onset type
2 diabetes. The association was independent of known risk factors for type 2 diabetes and was not attributable to treatment
for sleep disturbance. Sleep disturbance at baseline was unlikely due to complications or disability from the treatment of
diabetes because we excluded subjects with known diabetes at baseline. A possible explanation is that an increased sympathetic
nervous activity associated with sleep disturbance (5) causes glucose intolerance (6) and increases the risk of type 2 diabetes. Physicians may need to pay more attention to patients with sleep disturbance
because it may indicate a higher risk for type 2 diabetes.